1. Field of the Invention
The present invention relates to a cathode ray tube (CRT), and more particularly, to a dynamic vibration absorber in a cathode ray tube, which can attenuate a vibration of a shadow mask caused by an external impact effectively, and is of a type that exhibits almost no variation of a natural frequency for a variation of a temperature.
2. Background of the Related Art
A structure of a related art CRT will be explained, with reference to FIG. 1.
At a rear end of a panel 1 having a R, G, B fluorescent film coated thereon, there is a funnel 2 having an electron gun sealed therein for emitting an electron gun 6 welded thereto. There is a shadow mask 3 fitted to an inside surface of the panel 1 having a plurality of slots for passing the electron beam, and there are a deflection yoke 5 and a magnet 10 fitted to an outside surface of the funnel 2. Also, there is a reinforcing band 11 on an outside surface of the panel 1 for preventing breakage of the CRT from an external impact.
The shadow mask 3 is fitted to have a gap to the inside surface of the panel 1 by a main frame, and the main frame 7 is fastened to the panel 1 by springs 8. Also, there is an inner shield 9 fitted the main frame 7 for shielding the CRT from an external geomagnetism so that the CRT is affected less by the geomagnetism.
Referring to FIGS. 2 and 3, a shadow mask assembly will be explained.
The shadow mask 3 is welded to one pair of main frames 7 under tension. The shadow mask 3 is liable to vibrate by an external vibration, such as from a speaker. The vibration causes a color error in forming a picture by means of the electron beam, to deteriorate the picture. Therefore, a vibration absorber is provided to the shadow mask 3 for absorbing the vibration on the shadow mask 3. In detail, there are sub-frames 12 between the one pair of the main frames 7, and there are damper springs 14 fitted to the sub-frame 12, and there is a damper wire 13 between the damper springs 14. When a tension is applied to the damper wire 13 by using the damper spring 14, the damper wire 13 is pressed onto the shadow mask 3, to prevent vibration of the shadow mask 3. There are about three lines of the damper wires 13, for prevention of vibration.
However, the related art vibration absorber of the damper wire has the following problems.
Since the damper wire 13 has a very thin diameter of approx. 30 μm, the damper wire is highly susceptible to breakage during fabrication of the CRT. Also, there are cases when the damper wire 13 is broken during use after the CRT is sold to the user. Because an inner space of the CRT is under vacuum, the broken damper wire moves therein, to show a shadow of the broken damper wire on the screen forming a defective picture, of which repair is impossible. Moreover, the expensive precision apparatus for handling the fine damper wire is a factor that makes the production cost high.
In order to solve the foregoing problems, a dynamic vibration absorber may be utilized. A principle of the dynamic vibration absorber will be explained, with reference to FIGS. 4 and 5. FIG. 4 illustrates an example a dynamic vibration absorber is applied to a one degree of freedom system.
An object system 1 S1, of which vibration is intended to be reduced can be represented with a mass m1 and a spring constant k1. When an external force exciting a vibration at a frequency w is applied to the system 1 S1, a vibration with a frequency w occurs at the system 1 S1. For attenuating the vibration of the system 1 S1, a system 2 S2 having a natural frequency w is provided to the system 1 S1. The system 2 S2 may also be represented with a mass m2 and spring constant k2. In this system, the vibration of the system 1 S1 is transmitted to the system 2 S2, such that, not the system 1 S1, but the system 2 S2, vibrates. Accordingly, an effect of reducing the vibration of the system 1 S1 can be obtained. The system 2 S2 provided to reduce the vibration is called as a dynamic vibration absorber.
With regard to the dynamic vibration absorber, it is important how much well the natural frequency of the system 2 S2 is tuned to the excited frequency. If the excited frequency and the system 2 S2 are not well tuned, there is no vibration attenuation effect at all, and, contrary to this, the natural frequency of the system 1 S1 is increased.
For correcting the foregoing disadvantage of the dynamic vibration absorber, damping means, i.e., a damper c2 may be added to the system 2 S2. An appropriately designed damper c2 fitted to the system 2 S2 can provide a vibration attenuation effect even if the tuned slightly inaccurately.
FIGS. 6 and 7A-7D illustrate perspective views each showing a dynamic vibration absorber disclosed in U.S. Pat. No. 4,827,179, wherein a dynamic vibration absorber of a system with one degree of freedom is applied to a shadow mask of a system with multiple degrees of freedom. The U.S. Pat. No. 4,827,179 discloses multiple dynamic vibration absorber applied to a shadow mask that has a natural frequency varied with a temperature of a screen during operation of the CRT, and designed only to attenuate a first order vibration of the shadow mask.
FIG. 6 illustrates a dynamic vibration damper without a damper. The related art dynamic vibration absorber has a problem in that, though attenuation of vibration is good at a certain temperature owing to good tuning, the attenuation of vibration becomes poor sharply due to no provision of a cantilever matched to the first order of natural vibration of the shadow mask. Accordingly, referring to FIGS. 7A-7D, the U.S. Pat. No. 4,827,179 discloses addition of a damper to the dynamic vibration absorber for overcoming a problem of mis-tuning. However, the first order natural frequency of the shadow mask varies with the screen temperature more than 100 Hz, it is difficult for the multiple vibration absorber to cover such a great variation of the frequency.
In the meantime, the U.S. Pat. No. 4,827,179 discloses tuning the natural frequency by using change of a length of cantilever, fitting the tuned cantilevers to a rigid bracket which is in turn fitted to a non-effective surface of the shadow mask. However, the foregoing method has the following problem.
The individual fitting of the plurality of cantilevers to the bracket in the U.S. Pat. No. 4,827,179 requires much time, causing difficulty in fabrication. Moreover, the rigid bracket between the shadow mask and the cantilever impedes a smooth transmission of a vibration energy, that makes the vibration attenuation effect poor.
Moreover, referring to FIGS. 7A and 7D, the U.S. Pat. No. 4,827,179 discloses the means for supplementing a damping capability by friction or collision, i.e., a damper, provided as a separate member, that is not suitable for mass production because of a high production cost, and defects in an impact test of the CRT.